Most rubbery materials are electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10 to 10.sup.5 .OMEGA.-cm are conductive rubbers which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its heat resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents like the other rubbery materials.
The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and monofilaments surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing these conductive agents, the volume resistivity of silicone rubber can be reduced to the order of 10.sup.10 to 10.sup.-3 .OMEGA.-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber. Particularly when highly conductive silicone rubbers having a resistivity of about 10.sup.5 .OMEGA.-cm or lower are desired, carbon black and metal powders such as silver and nickel are used, with carbon black being often used in view of cost.
However, when a silicone rubber composition having conductive carbon black such as acetylene black blended therein is molded and processed into a length of article such as sealing members, gaskets, and rolls as by extrusion molding followed by vulcanization, strict limits are imposed on the vulcanization system.
More particularly, in the case of organic peroxide vulcanization, for example, acyl peroxides such as benzoyl peroxide and 2,4-dichlorobenzoyl peroxide are commonly used with silicone rubber compositions for enabling HAV. However, when such acyl peroxides are added to silicone rubber systems containing carbon black, the carbon black retards vulcanization, failing to provide satisfactory molded products. Other useful vulcanization agents are alkyl peroxides such as dicumyl peroxide and di-t-butyl peroxide. When such alkyl peroxides are added to silicone rubber systems containing carbon black, the systems can be compression molded. In the case of extrusion atmospheric hot air vulcanization, however, the same systems fail to provide satisfactory products because of the influence of oxygen in air resulting in short surface vulcanization.
For extrusion HAV of carbon black-loaded silicone rubber compositions, a common prior art approach to avoid such difficulty is by addition vulcanization. In accordance with this approach, an organopolysiloxane containing an alkenyl group is cured by adding thereto an organohydrogenpolysiloxane having a silicon-attached hydrogen atom capable of addition reaction with the alkenyl group and a platinum series addition reaction catalyst. This addition reaction system, however, suffers from serious problems associated with molding since it is limited in shelf life and can be prevented from curing by poisons such as amines, sulfur and tin.
In molding conductive silicone rubbers into electromagnetic radiation shielding gaskets, building gaskets, business machine use conductive rolls (such as electrostatic rolls, transfer rolls, developing rolls, paper feed rolls, and fixing rolls), and conductive calendered stocks to be shaped into zebra connectors, the molding process in accordance with the conventional HAV is disadvantageous in moldability and product quality.
To overcome these problems, the inventors previously proposed Japanese Patent Application No. 416974/1990 or U.S. patent application Ser. No. 07/812,285, now U.S. Pat. No. 5,299,573, or European Patent Application No. 92312013.5 which claims a conductive silicone rubber composition comprising
(A) an organopolysiloxane of the general formula (1): EQU R.sub.a.sup.1 SiO.sub.(4-a)/2 ( 1) PA1 (B) conductive carbon black, and PA1 (C) an organic peroxide of the general formula (2): ##STR1## wherein X is a group of the following formula (3) or (4): EQU --(CH.sub.2).sub.n -- (3) ##STR2## wherein n is an integer of 2 to 8, PA1 R.sup.2 and R.sup.3 are each a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group of the following formula (5): EQU --SiR.sub.3.sup.4 ( 5)
wherein R.sup.1 is independently selected from substituted or unsubstituted monovalent hydrocarbon groups and letter a is a positive number of 1.90 to 2.05,
wherein R.sup.4 is a methyl, ethyl or phenyl group.
This conductive silicone rubber composition can be satisfactorily molded and vulcanized by extrusion molding or calendering followed by HAV, offering conductive silicone rubber having improved physical properties.
Continuing research works, we found that the above-defined conductive silicone rubber composition exhibited satisfactory physical properties and workability immediately after its preparation, but with the lapse of time, experienced an increase of plasticity and scorching phenomenon. There is a need for increasing the shelf stability of such a composition.